1. Field of the Invention
This invention generally relates to Single Carrier Frequency Division Multiple Access (SC-FDMA) Multiple-Input Multiple-Output (MIMO) communications and, more particularly, to soft interference cancellation (SIC) using either a serial or parallel channel equalization process.
2. Description of the Related Art
UL: Uplink
LTE: Long Term Evolution (3GPP term for next-generation cellular standard)
OFDMA: Orthogonal Frequency Division Multiple Access
SC-FDMA: Single Carrier Frequency Division Multiple Access
MIMO: Multiple-Input Multiple-Output
MU-MIMO: Multi-user Multiple Input Multiple Output
SU-MIMO: Single-user Multiple Input Multiple Output
SISO: Single Input Single Output
SIMO: Single Input Multiple Output
SIC: Soft Interference Cancellation
TO: Timing Offset
ACK: Acknowledgement
NACK: Negative Acknowledgement
DTX: Discontinued Transmission
CFO: Carrier Frequency Offset
AWGN: Additive White Gaussian Noise
eNodeB: Enhanced NodeB (LTE base station)
UE: User Equipment
UCI: Uplink Control Information
RB: Resource Block
LTE and LTE-A use single-carrier frequency division multiple access as their uplink transmission technology. In LTE, multiple users can be spatially multiplexed on the same frequency-time resources, with each user sending a single spatial stream (MU-MIMO). In addition to this, LTE-A also supports SU-MIMO. Each user can send up to multiple spatial streams through its multiple transmit antennas.
FIG. 1 is a diagram depicting a Multiuser MIMO (MU-MIMO) wireless communication system (prior art). Multiple users can transmit data simultaneously at the same frequency to a multi-antenna base station, resulting in increased aggregate cell throughput. There is a need to decouple data streams from different users via MU-MIMO equalization, which requires MU-MIMO channel estimation. SU-MIMO (not shown) is similar except that a single user transmits via multiple antennas.
SC-FDMA is a multi-user version of a single carrier frequency domain multiplexing modulation scheme. SC-FDMA can be viewed as a linearly precoded OFDMA scheme, henceforth LP-OFDMA. Or, it can be viewed as a single carrier multiple access scheme. Just like in OFDM, guard intervals with cyclic repetition are introduced between blocks of symbols to efficiently eliminate time spreading (caused by multi-path propagation) among the blocks. In OFDM, a FFT is applied on the receiver side on each block of symbols, and inverse FFT (IFFT) on the transmitter side. In SC-FDMA, both FFT and IFFT are applied on the transmitter side, and also on the receiver side.
In OFDM as well as SC-FDMA, equalization is achieved on the receiver side after the FFT calculation, by multiplying each Fourier coefficient by a complex number. Thus, frequency-selective fading and phase distortion can be combated. The advantage is that FFT and frequency domain equalization requires less computation power than conventional time-domain equalization.
In MIMO systems, a transmitter sends multiple streams by multiple transmit antennas. The transmit streams go through a matrix channel which consists of all paths between the transmit antennas at the transmitter and receive antennas at the receiver. Then, the receiver gets the received signal vectors by the multiple receive antennas and decodes the received signal vectors into the original information. A narrowband flat fading MIMO system is modeled as:y=Hx+n
where y and x are the receive and transmit vectors, respectively, and H and n are the channel matrix and the noise vector, respectively, where x is a Mt×1 vector, where Mt is the number of transmit antennas, and where y and n are Mr×1 vectors.
FIG. 2 is a diagram depicting an exemplary MIMO receiver (prior art). Channel estimation is needed in multi-user and single-user MIMO receivers to separate different spatial streams and/or user signals via equalization. Of special interest is OFDMA and SC-FDMA multi-user MIMO channel estimation with a single spatial stream per user (e.g., LTE uplink). After cyclic prefix (CP) removal and a fast Fourier transform (FFT), the input to the channel estimator block is the received frequency domain signal of reference symbols from Mr number of receive antennas. The outputs are channel responses in the frequency domain from user u (1≦u≦U) to antenna m (0≦m≦Mr−1) are demodulated (demod) and decoded.
With respect to MU-MIMO channel estimation for OFDMA/SC-FDMA, user reference signals with different cyclic shifts are orthogonal across a number of tones in ideal scenarios (no timing offset and low delay spread). In this case, channel estimation for each user is decoupled. In practice, orthogonality is destroyed because of different user timing offsets and/or medium to high delay spreads. As noted in the patent application entitled, MULTIUSER MULTIPLE INPUT MULTIPLE-OUTPUT (MU-MIMO) CHANNEL ESTIMATION FOR MULTICARRIER COMMUNICATIONS, invented by Ravi Narasimhan et al., Ser. No. 12/782,066, the loss of orthogonality between the reference signals caused by propagation medium distortion errors may be compensated for by finding a channel estimate across the plurality of adjacent subcarrier frequencies for each multicarrier signal channel, in response to assuming a linear phase rotation for each channel across the plurality of adjacent reference signal subcarriers, and a constant amplitude for each channel across the plurality of adjacent reference signal subcarriers. More explicitly, the assumption of linear phase rotation and constant amplitude permits the use of one of the following DoA algorithms: classic beamforming, Capon beamforming, MUSIC, ESPRIT, alternating projection, or simplified projection.
In order to detect and decode all the multiplexed spatial streams, either from a single user or multiple users, the number of the receive antennas at the eNodeB has to be at least the total number of the spatial streams. A typical receiver uses a MMSE equalizer, which applies a feed-forward weight to the received signal vectors to minimize the post-equalized symbol-wise mean square error (MSE).
However, it has been shown that for linear receivers there is a performance loss of SC-FDMA uplink in frequency selective fading channel against OFDMA uplink. This is due to the “noise enhancement” from the DFT kernel of SC-FDMA spreading out the noise on deep faded tones. Further, conventional models limited to MU-MIMO, SU-MIMO, or SU-SIMO do not use the feedback of either the desired user/stream or interfering user/stream to improve performance.
It would be advantageous if a SIC equalizer could use the soft symbol outputs from both the desired and interfering user/stream's decoders to improve the performance by not only canceling the interference, but also by providing better equalization for SC-FDMA uplink in frequency selective channel. Therefore, even when there is only a single user single stream uplink, the SIC equalizer degrades to a turbo equalizer, and still provides performance gain.